This invention relates generally to conveyor apparatus and, more particularly, to double-member or rail conveyor apparatus, such as retarding roll chain conveyors or twin-belt conveyors on which workpiece carriers are supported on the longitudinal conveyor rails or members for conveyance thereon.
Double rail conveyor apparatus of this type are well known. One problem in the operation of such conveyor apparatus is in the return of the workpiece carriers to their starting positions. Although this is generally not a problem in carousel-type conveyors in which a conveyor belt or the like circulates in a closed path in a single plane, such conveyors cannot be used in all applications due to their large space requirements.
Conveyor apparatus have been suggested wherein the workpiece carriers are supported on longitudinal rails, such as belts, in such a manner that upon reaching the end of their run, they are turned upside-down and returned to their starting positions on the lower run of the belts. Such apparatus, however, can be used for only certain specific applications and are not suited for conveying large, heavy pallets or the like since the pallets or workpiece carriers are too heavy to be conveyed back to their original starting point on the lower run of the conveyor belts.
For these reasons, it has been conventional to accomplish the return of the workpiece carriers being conveyed in a longitudinal direction to their starting positions by providing lifting devices, either at the ends of the conveyor or at intermediate locations along the conveyor, which remove the workpiece carriers from the conveyor in a lateral or transverse direction whereupon the workpiece carrier is transferred by lift apparatus to a level above or below the conveyor where it is again shifted laterally and placed on a second conveyor for the return conveyance. Besides being extremely complex in construction, especially where the workpiece carriers must be transferred to the return conveyor at several intermediate locations, a significant amount of space is required on the sides of the conveyor to accomplish the lateral shifting of the workpiece carriers.
In another conventional conveyor apparatus used in transporting articles having rigid bottom plates with sliding carriers, the carriers are supported in closely adjacent relationship on upper and lower belts so as to push each other along a horizontal conveyance path. Lifting conveyors are provided at both ends of the conveyor for transferring the sliding carriers from one conveyor belt to the other. This arrangement has the drawback that the carriers are moved forwardly with one carrier pushing against the next closely spaced adjacent carrier, from the first to the last carrier thereby increasing the possibility of empty carriers being conveyed. The carriers cannot be spaced from each other as is required with loose chain storage systems.
Another conventional conveyor construction comprises roller conveyors provided with lifting stations at the starting and ending points thereof. Such construction has the drawback that drive elements are required to move the workpiece carriers to the lifting stations from the conveyor and from the lifting stations to the conveyor. Another serious drawback of such a construction is that in the case where several loading/unloading sites are required between the starting and ending points, it is necessary to provide several lifting stations and associated drive stations between the lifting stations at each site.
Conveyors are also known which utilize workpiece carriers whose width is adjustable so that the carriers can be moved vertically between lateral longitudinal conveyors with an adjustable lift to conveyor planes at different levels. Such constructions, however, have the drawback that each individual workpiece carrier must have lateral bearing rails fastened to it or transversely shiftably supported guide pins which are moved apart and together through an adjusting device.
Moreover, conveyor apparatus of this type have numerous moving parts and are complicated and expensive in construction and have the additional disadvantage that, for example, with conveyor systems forming a square or which are arranged at other angles with respect to each other, considerable modifications must be made to redirect the workpiece carriers around the corners into a new conveying direction.
Furthermore, it is not possible using this construction to pivot the workpiece carriers around their central vertical axes into appropriate orientations, such as for loading or unloading or for simple assembly operations.